Diatomaceous earth products are obtained from diatomaceous earth (also called “DE” or “diatomite”), which is generally known as a sediment enriched in biogenic silica (La, silica produced or brought about by living organisms) in the form of siliceous skeletons (frustules) of diatoms. Diatoms are a diverse array of microscopic, single-celled, golden-brown algae generally of the class Bacillariophyceae that possess an ornate siliceous skeleton of varied and intricate structures comprising two valves that, in the living diatom, fit together much like a pill box.
Diatomaceous earth may form from the remains of water-borne diatoms and, therefore, diatomaceous earth deposits may be found close to either current or former bodies of water. Those deposits are generally divided into two categories based upon source: freshwater and saltwater. Freshwater diatomaceous earth is generally mined from dry lakebeds and may be characterized as having a low crystalline silica content and a high iron content. In contrast, saltwater diatomaceous earth is generally extracted from oceanic areas and may be characterized as having a high crystalline silica content and a low iron content.
In the field of filtration, methods of particle separation from fluids may employ diatomaceous earth products as filter aids. The intricate and porous structure unique to diatomaceous earth may, in some instances, be effective for the physical entrapment of particles in filtration processes. It is known to employ diatomaceous earth products to improve the clarity of fluids that exhibit turbidity or contain suspended particles or particulate matter.
Diatomaceous earth may be used in various embodiments of filtration. As a part of pre-coating, diatomaceous earth products may be applied to a filter septum to assist in achieving, for example, any one or more of: protection of the septum, improvement in clarity, and expediting of filter cake removal. As a part of body feeding, diatomaceous earth may be added directly to a fluid being filtered to assist in achieving, for example, either or both of: increases flow rate and extensions of the filtration cycle. Depending on the requirements of the specific separation process, diatomaceous earth may be used in multiple stages or embodiments including, but not limited to, in pre-coating and in body feeding.
Diatomaceous earth filter aids may also comprise metals, such as iron, that may be soluble in the liquid media being filtered. When those diatomaceous earth filter aids are used to filter liquids, the metals may disassociate from the diatomaceous earth filter aid and enter the liquid media. In many applications, this increase in metal content of the liquid media may be undesirable or even unacceptable. For example, when diatomaceous earth filter aids may be used to filter beer, a high level of iron dissolved in the beer originating from the filter aid material may adversely affect sensory or other properties, including but not limited to taste and shelf-life. Other non-diatomaceous earth filter aids may suffer from a similar metal-leaching effect. Thus, the brewing industry has long recognized the importance of reducing iron dissolution in beer and has sought out filter aids with increasingly lower beer soluble iron (BSI) contents.
The brewing industry has developed at least two protocols to measure the BSI of diatomaceous earth filter aids. The European Beverage Convention (EBC) method contacts a potassium hydrogen phthalate solution with the filter aid and then analyzes the filtered liquid for iron content. The American Society of Brewing Chemists (ASBC) method contacts a sample of beer with the filter aid and then measures the resulting iron content in the liquid. Other protocols may also be known and used.
The EBC method uses an international method for determining the beer soluble iron content of a filter aid. More specifically, the EBC method uses, for example, about 10 g/L solution of potassium hydrogen phthalate (KHP, KHC8H4O4) in distilled water as the extractant. In the EBC method, about 5 g of a filter aid sample is mixed with about 200 mL of the KHP solution for about 2 hours using a magnetic stirrer so that the filter aid remains in suspension during extraction. The resulting solution is then filtered immediately through a filter paper, about the first 50 mL is discarded, and about the next 100 mL is collected for analysis. Extracts are then analyzed for iron concentration by the FERROZINE method, in which a FerroZine® reagent (disodium salt of 3-(2-pyridyl)-5,6-bis(4-phenylsulfonic acid)-1,2,4-triazine, C20H14N4O6S2, available from Aldrich) may be used as the color development reagent. Typically, the FerroZine® reagent is prepared by dissolving about 5 g of FerroZine® reagent in about 1000 mL of an ammonium acetate/acetic acid buffer with a pH of about 4.3. The FERROZINE method uses about a 25 mL portion of the extractant, and treated with about 25 mg of ascorbic acid (C6H8O6) to reduce dissolved iron ions to the ferrous (Fe2+) state, thus yielding a colored iron complex with the FerroZine® when color is developed by adding about 1 mL of the FerroZine® reagent. After about 30 minutes, the absorbance of the resulting sample solution is compared to a standard calibration curve. Absorbance is measured at about 565 nm using a spectrophotometer and compared against the standard to measure BSI.
The ASBC method may measure BSI content by placing about 5 g sample of a filter aid material in about 200 mL of de-carbonated beer (for example, BUDWEISER® from Anheuser-Busch, St. Louis, Mo., USA) at room temperature and swirling the mixture intermittently for an elapsed time of about 5 minutes and 50 seconds. The mixture is then immediately transferred to a funnel containing about 25 cm diameter filter paper, from which the filtrate collected during about the first 30 seconds is discarded. Filtrate is collected for about the next 150 seconds, and an about 25 mL portion is treated with about 25 mg of ascorbic acid (C6H8O6) to reduce dissolved iron ions to the ferrous (Fe2+) state, thus yielding a sample extract. Color is then developed by adding about 1 mL of about 0.3% (w/v) 1,10-phenanthroline and, after about 30 minutes, the absorbance of the resulting sample solution is compared to a standard calibration curve. The calibration curve is prepared from standard solutions of known iron concentrations in beer. Untreated filtrate is used as a method blank to correct for turbidity and color. Absorbance is measured at about 505 nm using a spectrophotometer and compared against the standard to measure BSI.
Many methods have been developed to reduce the content of BSI in diatomaceous earth filter aids. One such method is crude diatomaceous earth filter aid selection. Some deposits of diatomaceous earth naturally contain less iron than diatomaceous earth material from deposits in other locations. Crude selection alone, however, may not be sufficient to supply the brewing industry and other applications with reduced BSI or iron-content diatomaceous earth.
Another method known to reduce BSI content in diatomaceous earth is the process of calcination. Calcination generally involves heating diatomaceous earth at a high temperature, for example in excess of about 900° C. Calcination may reduce the presence of organics and volatiles in the diatomaceous earth and induce a color change from off-white to tan or pink. Because of agglomeration and sintering of fine particles, such as fine diatomaceous earth fragments and clays, during calcinations, the surface area of diatomaceous earth is generally reduced. In one example, the DE surface area is reduced from about 25 m2/g to about 45 m2/g for the natural diatomaceous earth to less than about 10 m2/g, thus leading to the reduction of soluble metals, including but not limited to iron.
Additionally, the beer soluble iron content of a diatomaceous earth filter aid, particularly the BSI as measured by ASBC method, may decrease naturally and gradually with time after calcination. Surface re-hydration by humidity in the ambient air, for example, is one mechanism of BSI reduction. Achieving BSI reduction naturally, however, may take months, and the results may fluctuate with seasons and crude selection.
Apart from or in addition to crude selection and calcination process control, chemicals may be applied to filter aids to reduce BSI content. Chemical processes include, for example, acid-washing and/or leaching with chelating solutions such as EDTA or citric acid. Although such methods can be somewhat effective to reduce surface soluble metals, the processes are usually expensive. In addition, highly soluble metals may re-emerge in the filter aids if abundant refreshed surfaces reappear during chemical or mechanical processing. Furthermore, in some applications, chemical treatments may be undesirable or unacceptable. For instance, in applications regulated by the U.S. Food and Drug Administration, water is the only chemical allowed in the post-calcination processing of filter aids without the chemical undesirably being labeled as an additive.
Water treatment may comprise, for example, spraying water to the bottom of a bulk container comprising filter aids or into bags during packaging. Water treatment at higher temperatures is known to accelerate the BSI reduction process, yet because water treatment generally occurs in an open container, the temperature of the treatment cannot be higher than the boiling point of water. Typical water treatments may include spraying and mixing water into a diatomaceous earth filter aid product while the product is hot (for example, at a temperature ranging from about 150° F. to about 200° F.). The treated product may be held in containers, such as bins and rail cars, until the BSI is reduced to the desired level. Water treatments may also comprise the use of steam treatment. However, the BSI reduction effects of water treatments are often limited in BSI reduction and, therefore, water-treatment cannot be used to effectively treat filter aids that may have relatively high BSI levels, such as some diatomaceous earth.
Although calcination and water treatment may generally reduce the BSI content of diatomaceous earth filter aids and are generally effective for straight calcined diatomaceous earth products, in which reduced BSI is generally measured by the ASBC method. However, when the BSI is determined by the EBC method (e.g., using about 1% KHP as the extractant and extracting for about 2 hours), the BSI in the filter aids is much higher than determined by the ASBC method, in which a beer is used. Additionally, water treatment generally does not generate a measurable reduction in BSI if it is determined by the EBC method. Generally, the BSI of a diatomaceous earth filter aid as measured by the EBC method is several times higher than the BSI as measured by the ASBC method, which is generally due to the fact that KHP is a much stronger chelating agent for iron than the chelating compounds present in beer.
In addition, when diatomaceous earth is calcined with an alkali flux agent, such as soda ash, the flux-calcined products generally show higher BSI than straight calcined products, generally because of the surface iron liberation by alkali ions. The BSI levels are also generally affected by the calcination intensity that the products received; intensively heated products generally show less BSI than the less intensively heated products.
EBC protocols and standards are widely used by the brewing industry, and the diatomaceous earth filter aids used for beer filtration are desired to have low beer soluble iron as determined by the EBC method. Therefore, there exists a need for a low soluble metal containing diatomaceous earth filter aid product, as well as an inexpensive and effective method for reducing the amount of soluble metals in diatomaceous earth crudes, that may acceptably be used in applications requiring low metal content or dissolution. In particular, there exists a need for a low soluble metal containing diatomaceous earth filter aid product, as that soluble metal content is determined by an EBC method. Applicant has surprisingly found that such a diatomaceous earth product containing reduced soluble metal levels may be achieved by treating diatomaceous earth with at least one surface metal blocking agent, before at least one thermal treatment.